Capacitive multi-touch screens have been more and more widely used in handheld touch devices such as mobile phones and tablet PCs due to their unique advantages in tactile experience, light transmission rate and durability.
The basic detection principle for a capacitive multi-touch screen is briefly discussed below. The capacitive multi-touch includes a capacitive touch sensor and a touch controller connected to the touch sensor. To detect multiple touch points, the touch sensor includes a plurality of touch detection nodes arranged in an array. If the detection nodes are distributed over the touch sensor plane in m rows and n columns, they form an m*n detection node array as shown in FIG. 1A. By sampling the detection node array, the touch controller can obtain a corresponding m*n sampled data array. In practice, the sampled data in an array obtained by one time sampling is termed as one frame of sampled data. In order to detect a touch event, a reference data array is needed, which is established based on steady sampled data that are sampled under no touch event condition and corresponds to each detection data, as shown in FIG. 1B. According to the detection principle of capacitive touch screens, it can be easily understood that the varying portion of the current sampled data with respect to the reference data includes detected touch information.
Referring to FIG. 1C, in order to judge a touch event, the touch detection data of each sampling point is calculated, i.e. Dij=Sij−Rij, thereby obtaining a current detection data array or detection data frame, as shown in FIG. 1D. Assuming that a direction of variation of the touch detection data caused by the touch event is positive, the intensity of the touch signal can be measured according to amplitude of the positive variation. In practice, considering that amplitude of the detection data Du has a certain degree of jitter, a specific method of judging a touch event according to the detection data Dij is as follows. According to a dynamic range of the actual detection data of the touch screen, an appropriate touch event detection threshold Ht is selected such that the detection data with amplitude greater than this threshold is considered as a theoretically reliable touch event. That is, when Dij>Ht, it can be determined theoretically that a touch event is detected at this touch detection node. In a group of detection data illustrated in FIG. 1E and FIG. 1F, the detection data Dx3, Dx and Dx4 show such scenarios in which amplitude of the detection data varies because of the touch event and the amplitude variation goes beyond the touch detection threshold Ht. These detection data indicate the occurrence of a touch event. In addition, the detection data Dx3, Dx and Dx4 as well as the detection data distributed therearound can be used to calculate a specific location of the touch point.
FIG. 2 illustrates a general flow chart of touch screen data processing. An initialization stage presets various thresholds and sets initial values of various counters and timers. A touch detection data sampling and reference updating stage obtains a frame of detection data needed for touch detection and updates the reference data at an appropriate time. A touch event analysis stage calculates a location of each valid touch point on the touch screen according to the processing to the touch detection data. A touch detection result output stage outputs information such as, the location and movement trajectory of the detected touch point, to a processor of the touch device.
The above description is only about the basic touch detection principle. With respect to the capacitive multi-touch screen, the touch detection in practice applications is far more complicated. One important issue is that, the reference data of the touch screen need to be reliably and timely updated using new detection data after power on and during later using process; otherwise, reliable touch detection results cannot be continuously obtained from the detection data Dij. However, as already known in the art, the touch detection data obtained by the capacitive multi-touch screen in at least the following situations can easily experience abnormity, such that the reference data update may not be reliable.
First, if there is a touch on the touch screen during power on stage, after the first reference data update, the detection data obtained at this touch location probably becomes a data with a direction opposite to a normal detection data and with large amplitude.
Second, the capacitive multi-touch screen detection is based on mutual capacitance or projective capacitance detection principle, and the detection process involves the coupling capacitance between the body of the user and the touch device. If the user performs a multi-touch, under some specific conditions (e.g. when the coupling between the body of the user and the touch device is very weak), it may be caused that negative and large amplitude touch detection data may occur at some specific locations on the touch screen.
Third, water drops or water film on the touch screen surface can also severely affect the touch detection data, which may make the data deviate from normal condition and easily result in the negative and large amplitude touch detection data.
Detection data Dx6, Dx7 and Dx8 in FIG. 1F have a direction opposite to the direction of the usual touch event detection data (e.g. Dx3, Dx and Dx4), and the amplitude of Dx7 is greater than Ht. Such abnormal detection data may easily occur in the above situations. Practical application of the touch screen is far more complicated and unpredictable. Therefore, there is high possibility of the abnormity occurring in one frame of the detection data. Under the condition that such abnormal detection data may occur at any time, there may often be errors in the reference data update processing—once the negative and large amplitude detection data are used in the reference update, the consequence is often that, after the reference data is updated, the touch screen “detects” a false touch event, i.e. a static “touch point.” In addition, these false static touch points prevent further update of the reference data, which may make the touch device and system fall into disorder and go out of control.
The following measures have been widely taken to address these issues.
First, better touch screen materials are used, such as, using toughened glass rather than thin film, and fabrication precision of the touch sensor is increased.
Second, the area of a grounding portion of the touch device is increased, such that the coupling capacitance between the user and the touch device is increased and becomes more steady.
However, the first measure increases the fabrication cost—the strict fabrication process makes it difficult to reduce the defect rate and, considering the high material cost, the first measure is not suitable for massive use. The second measure attempts to use a larger grounding area to improve the touch detection performance. This measure can bring good effect to touch devices with relatively large size, such as, tablet PCs. However, to a small device such as a mobile phone, even if a casing of the small device is made from metal, the second measure can bring only limited improvement.